Switching converter is widely used recently in portable electronic device due to high efficiency, fast transient response and small size. However, it is easy to produce surge current during startup of a switching converter, and an output voltage may overshoot to damage the switching converter and other devices. As a result, a soft start circuit is needed.
A traditionally method to implement soft start of the switching converter is to provide a ramping up soft start reference signal during startup through charging a soft start capacitor by a current source, and the output voltage follows the soft start reference signal and increases slowly. However, the soft start capacitor usually has a large capacitance and is not convenient to be integrated, which will increase size and cost of the switching converter. Another method to implement soft start of the switching converter is to design a customized integrated soft start circuit. However, one disadvantage of this method is that the soft start time period is fixed and inflexible.